KABUL, Afghanistan - Afghan and coalition troops have found and destroyed more than two tonnes of marijuana in an abandoned school in southern Afghanistan.

Officials say the marijuana -- stored in stacks more than half a metre tall -- filled several rooms of the former school in the Arghistan district of the southern Kandahar province.

Afghanistan is the world's largest producer of opium, the main ingredient in heroin.

But as the U.S. and other western countries try to help Afghanistan stamp out its poppy fields, an increasing number of farmers have turned to marijuana, which is receiving less attention from authorities.

In Zabul province, meanwhile, coalition forces say they killed four armed militants and detained five suspects during an operation Saturday.

They say the operation targeted a Taliban militant "known to traffic weapons and co-ordinate roadside bomb attacks."

Violence has spiked in Afghanistan the last two years, and some 65,000 international troops, including about 2,500 Canadians, are now in the country.